[Objective] This study aims to address the issues of insufficient trajectory tracking accuracy and significant high-frequency chattering in permanent magnet direct current torque motor (PMDCTM) servo systems of laser trackers

which are caused by parameter variations

external disturbances

and nonlinear friction in conventional proportional-integral-derivative (PID) control. An nonsingular terminal sliding mode control (NTSMC) method with adaptive switching gain is proposed for PMDCTM position loop. [Methods] Firstly

a state-space representation based on angular error was developed using the electromechanical model of the azimuth axis motor in the laser tracker. Next

a fractional-order saturated function was selected as the sliding mode surface

and an adaptive switching gain law was designed by integrating adaptive control theory. This design effectively mitigated system uncertainties while avoiding singularities. Finally

the finite-time convergence of the control system was verified using Lyapunov stability theory

and the relationship between the designed sliding mode surface parameters and system convergence speed was analyzed. [Results] Comparative simulations and experiments were conducted using Matlab/Simulink software and enhanced programmable multi-axis controller platform. The results showed that

in the step response characterizing system rapidity

the PMDCTM position loop based on adaptive NTSMC (ANTSMC) reduced the convergence time by 41.2% compared to NTSMC and by 66.3% compared to PID control. In the application scenario of commutation of the tracker's azimuth axis motor

the ANTSMC-based PMDCTM position loop demonstrated a 37.2% reduction in peak transient tracking error compared to NTSMC and 76.9% reduction compared to PID control. Furthermore

the steady-state chattering was significantly reduced

thereby enhancing the system's ability to resist external disturbances. [Conclusion] The proposed ANTSMC method effectively improves the control accuracy of PMDCTM position loop

enhances the dynamic response performance of laser tracker servo systems

mitigates chattering phenomena

and demonstrates excellent robustness.